1. Field of the Invention
The present invention deals with the field of gas turbine technology. It relates to a method for operating a partially closed, turbocharged gas turbine cycle, and to a gas turbine system for carrying out the method.
2. Discussion of Background
It is known that gas turbines for performing work can be constructed on the basis of turbochargers, as are used, for example, to turbocharge internal combustion engines. One such example is the microturbines (of type MT100) which have been available for some years from ABB-Energie-Services, with an electrical power of 100 kW, which are intended to serve as small combined heat and power generation installations (cf. Schweizerische Technische Zeitschrift STZ No. 4/2002, pages 38 to 40 or ABB Review 3/2000, pp. 22-30).
Since gas turbines designed in this way have at most moderately cooled turbines, the hot-gas temperature which acts on the turbine and is generated by the combustion of a fuel in the combustion chamber arranged between compressor and turbine cannot meet the very high standard set by modern gas turbine construction. Accordingly, to achieve any worthwhile level of efficiency of power generation, it is necessary to incorporate a recuperator, as is the case with the abovementioned microturbine MT100. In the recuperator, heat is extracted from the hot turbine exhaust gases and used to heat the compressed combustion air before it enters the combustion chamber. This makes it possible to expect of efficiency of at least 30%.
Gas turbines constructed in this way result in a poor performance with regard to part-load efficiency, since the radial compressors used in turbochargers do not allow the intake mass flow to be controlled unless the throttling method, which is subject to very high levels of losses, were to be used. Consequently, the power has to be controlled by changing the turbine inlet temperature, which is highly unfavorable in terms of exergy.
A further drawback of a cycle configuration with a recuperator of this nature is firstly the low optimum pressure ratio with regard to efficiency, resulting in a low specific power based on the air flow. Secondly, the heat transfer on the low-pressure side of the recuperator is low, since the turbine exhaust gas is at only barometer pressure.
An unpublished earlier application in the name of the same Applicant has proposed a partially closed, turbocharged gas turbine cycle for conventional large gas turbines, in which an exhaust-gas turbocharger is connected on the low-pressure side of the recuperator, removes a partial stream from the cycle at a first suitable temperature level of the recuperator and expands it in the turbine of the exhaust-gas turbocharger, and also compresses intake ambient air in the turbine of the exhaust-gas turbocharger and feeds it to the cycle at a second suitable temperature level of the recuperator.